This invention relates to a circuit for starting and operating a high pressure arc discharge lamp, and more particularly to an ignitor or starting circuit which improves the starting characteristic of a "hot" dual ended high pressure discharge lamp.
Two conditions must be fulfilled in the starting process of a gas discharge lamp. First, the starting circuit must provide sufficient energy in the voltage pulse applied to the lamp electrodes. Second, the circuit must allow enough current to follow through in order to bring the electrodes to the proper emission temperature.
If the second condition is not satisfied, there will be an insufficient flow of current from the AC power supply source which will result in a lowering of the emission temperature of the lamp electrodes which are exposed to the high voltage pulses. This condition will cause sputtering of the electrodes with a concomitant decrease in the life expectancy of the discharge lamp.
It is generally known that a high voltage pulse of a value several times the lamp operating voltage is required to start or ignite certain types of high pressure gaseous discharge lamps such as a high-pressure sodium discharge lamp. U.S. Pat. No. 3,407,334 (10/22/68) in the name of O. G. Attewell, U.S. Pat. No. 3,963,958 (6/15/76) in the name of J. A. Nuckolls and U.S. Pat. No. 4,403,173 (9/6/83) in the name of W. Mayer describe three such ignitor circuits for high-pressure gaseous discharge lamps. In each of these patents a starting capacitor is serially connected with a resistor so that a high voltage starting pulse is obtained upon discharge of the capacitor into a step-up transformer via a voltage threshold device. The transformer has an output winding coupled to the electrodes of the high-pressure discharge lamp. After the lamp ignites, the high voltage discharge pulses are suppressed because the lower value of the lamp operating voltage prevents the starting capacitor from charging up to the breakdown level of the voltage threshold device.
Although the circuits described in the Nuckolls and Attewell patents are adequate for the ignition and operation of standard high-pressure sodium lamps, they are unreliable for the ignition of certain newer types of dual ended electrode halide lamps such as the HQI lamp manufactured by Osram GmbH and others. The older standard high-pressure sodium lamps generally require a starting voltage pulse in the range of 2.5 KV to 4 KV, whereas the HQI type of lamp requires a start voltage pulse in the range of 4 KV to 5 KV.
In these prior art circuits, a substantial dip in the lamp open circuit voltage occurs immediately after the generation of the high voltage ignition pulse. The dip in the lamp voltage is caused by the discharge of the start or surge capacitor when the voltage threshold device "closes". This reduction in voltage means less power is available at the lamp electrodes to sustain the arc discharge immediately after lamp ignition.
Another disadvantage of the above prior art pulse ignition circuits is that, after a momentary interruption of power to the high pressure discharge lamp, reignition of the lamp is not reliable because of the relatively low voltage that is available upon reapplication of the AC supply voltage. The applied high voltage ignition pulses will eventually ignite the lamp, but the low amplitude of the power frequency voltage may cause the lamp to hang up in a "low-glow" mode for a considerable period of time. This in turn will lead to deterioration of the lamp electrodes.
The Mayer patent provides a circuit for starting the newer type of higher pressure metal vapor discharge lamp under cold or warm conditions by means of a pulse superimposition ignition circuit. A starting capacitor and a resistor are serially connected to the AC supply voltage. This series circuit is also connected to the primary winding of a step-up pulse transformer. The transformer secondary winding is coupled to the lamp electrodes. A semiconductor voltage threshold element, such as a four-layer diode, is coupled to the capacitor so as to allow the capacitor to discharge via the transformer primary winding to induce a large number of high voltage pulses in the transformer secondary winding in each half cycle of the AC supply voltage. Mayer also provides an oscillatory circuit composed of an auxiliary capacitor and a serially connected damping resistor which form, together with a ballast choke, a series resonant circuit coupled to the primary winding of the pulse transformer so as to increase the lamp supply voltage during ignition. This ignitor was designed for use with reactor ballasts to ignite lamps which required higher R.M.S. starting voltages than were available from the 220 V power supply. The semiconductor switching element (four-layer diode) disconnects the series resonant circuit after lamp ignition so that in normal operation the lamp will only receive power from the AC supply voltage source.